Mathematical learning has an important role and is often prioritized in education. In K-16 education, algebra is one of the most vital mathematical content domains: it represents one of the top barriers for students pursuing a postsecondary education. Game-based learning has been effective in fostering classroom math learning environments that are collaborative and focused on conceptual understanding. Sandbox games provide open-ended learning environments where players can set their own goals and level of effort. As part of the project "Math Snacks," the team designed Agrinautica, a sandbox game to enable constructivist-informed early algebra learning. This article identifies design recommendations for creating meaningful sandbox games for learning, considering students' and teachers' needs. Researchers discuss the decisions to create a sandbox game and describe challenges inherent in math learning through sandbox-type gameplay. This study provides impact results from a large-scale study of users of the game, and shares recommendations for developing future sandbox learning games.